This invention relates to devices for sighting in firearms, and more particularly to devices for accurately positioning the firearm sight to correspond to the point of impact of projectiles expelled from said firearms.
Firearms are often equipped with sights which a person using the firearm uses to aim the firearm. The sights often comprise a telescope which allows the user a magnified view of his target. Such telescopes include reticles (cross hairs) observable while viewing a target through the telescope. The process of "sighting-in" the firearm consists of aligning the firearm sights, particularly the telescope cross hairs, so as to accurately correspond to the point of impact of projectiles fired from the firearm.
Each caliber of firearm (and even similar calibers made by different manufacturers) and bullet muzzle velocity results in a different bullet trajectory. All trajectories are nearly parabolic due to gravity. Thus, firearms are generally sighted-in for the muzzle velocity and distance of bullet travel expected in actual use. For example, a hunter will sight in his particular caliber rifle for shooting a particular bullet at a particular distance.
One method of sighting-in firearms is to shoot at the center of a target located at the desired distance. The point where the bullet actually strikes the target is noted, the firearm sights are adjusted, and the process is repeated until the desired accuracy is obtained.
Various devices have been designed to aid in sighting-in firearms. For example, U.S. Pat. No. 1,295,075 to Sheppard discloses a bore sighting device which mounts in the end of a firearm bore and sets up a line of sight parallel with the axis of the bore mounting segment of the device. Likewise, U.S. Pat. No. 3,112,567 to Flanagan describes a device mountable in the end of a rifle which includes marks to which the rifle telescope can be adjusted depending on the expected target distance. U.S. Pat. No. 3,744,133 to Fukushima discloses a collimating device which has a conical surface to center the device in various calibers of firearms. U.S. Pat. No. 4,090,305 to Cassidy also discloses a device to be mounted in the end of a gun barrel in an aid to sighting-in the sights on the gun.
One common problem with all of the above devices is that when used, the firearm is sighted-in on a device at the end of the barrel, only a foot or two at most from the gun sights. Therefore, any slight error made while sighting-in using the device will be greatly magnified when shooting at a target several hundred yards away. Even if the precision of alignment were .+-.0.01 inches for a device mounted 2 feet from the guns telescope, at 300 yards the precision would be .+-.4.5 inches, or about a 10 inch diameter circle, Thus, these types of prior art devices are typically used only as a first approximation, and actual target shooting is required to sight the firearm in with the desired degree of accuracy.
German Patentschrift No. 647,136 describes a device for checking the accuracy of the sighting mechanism of a gun. The device uses a prismatic telescope in an optical arrangement designed to view along the axis of bore to check the accuracy of the gun sighting device. A problem common to the disclosed device and the previously discussed devices is the lack of accuracy in lining up the device itself with the axis of the firearm bore. In the case of the German reference, the optical device is mounted in the bore of the gun on a mandrel which includes two spherical bearing surfaces. In order that such a mandrel could be inserted into and removed from the bore, there must be sufficient clearance between the inside of the bore and the bearing surfaces to allow such placement without harm to the inside of the bore. Any freeplay between the mandrel and bore will result in a slight angle between the axis of the bore and the axis of the optical arrangement mounted on the mandrel. The small moment of error thus present will affect the accuracy to which the gun can be sighted-in. Another disadvantage is that a separate mandrel must be used for each caliber.
The devices disclosed in the Cassidy and Fukushima et al. patents use a compressible member to expand within the bore in an attempt to hold the devices in line with the axis of the bore. The nature of the expandable means used, however, makes it very difficult to insure exact centering. The device disclosed in the Flanagan patent is mounted only at the mouth of the bore.